This invention relates to an antenna that is miniature, when compared to prior antennas of the same category. In particular, the antenna of the present invention will be useful for communications that use frequency bands in the mega Hertz (MHz) range or in the giga Hertz( GHz) range.
With the widespread proliferation of telecommunication technology in recent years the need for small size antennas has increased many fold. However, the solution is not so simple as arbitrarily reducing antenna size as this would result in a large input reactance and a deterioration in the radiation efficiency.
There is an unprecedented demand for compact electrically small antennas with moderate gain that are compatible with the recent revolutionary advances in the semiconductor industry. With the associated electronics being miniaturized, conventional antennas would not be acceptable to the end user. Reducing the physical size of the antenna and restricting it to a planar configuration has been the aim of antenna designers. However, most of the low frequency communication antennas currently operating in land, air and maritime mobile systems are of either low bandwidth or large size. Mobile antenna development is no longer confined to the design of small light weight antennas but it is more of a creation of a well defined electromagnetic configuration which can contribute significantly in signal processing and data communication in ill-defined and time varying environments. What is needed is an improved bandwidth for antennas of mobile communication systems that could lead to diversity in reception capability, reduction of multi-path fading, and selectivity of polarization characteristics, in addition to the fundamental increase in the speed of information transfer. Also needed is a small size antenna that can be implemented in a conformal configuration that is sleek and aesthetic and will fit in small handheld electronic equipment.
Prior art approaches to extending the bandwidth of conventional antennas have been pursued for few decades, but most of these are not conformal. One type of conformal antenna is the microstrip antenna. However, the microstrip antenna suffers from disadvantages, such as small bandwidth and low gain. Various approaches to improve the bandwidth of microstrip antennas include the use of multi-layer structures, parasitic elements, log periodic structures, shorting pins, and specially shaped patches. However, all these methods lead to fabrication difficulties and make the antenna configuration bulky, especially at lower frequencies. Although high dielectric substrates may reduce the size, the gain of the antenna is degraded by their use.
A type of pattern that is non-eucledian has been described in Fractal Geometry of Nature, 1983, by B. B. Mandelbrot. Mandelbrot contended that it is possible to describe many of the irregular and fragmented patterns in nature to full-fledged theories by identifying a family of shapes called xe2x80x9cfractalsxe2x80x9d. The geometric self-similarity of these patterns has been very enthusiastically followed in many fields of engineering (e.g., remote sensing, pattern recognition, signal processing, etc.). The self-similar nature of fractal patterns has been studied widely and is used in many fields of science and engineering, such as image processing and pattern recognition. Although a large number of fractal patterns have been described, one pattern, known as the Sierpinski gasket, is popular in engineering applications, such as finite element methods. For example, Pascal-Sierpinski gaskets have been used in finite element mesh generation for vibration problems with a significant reduction in the computation time and storage requirements. While analyzing the basic vibration properties, computation time and memory requirements in comparison to traditional meshing approaches, a new mesh generation based on geometric fractals offers much promise in significantly reducing storage requirements and computation time. The use of fractal structures to solve problems involved in array synthesis has been described in an article, Self-Similarity in Diffraction by a Self Similar Fractal Screen, IEEE Transactions Antennas Propagation, vol. Ap-35, pages 236-239, 1987 and in an article, On a New Class of Fractals:the Pascal-Sierpinski Gaskets, Journal of Applied Physics, Vol. 19, pages 1753-1759, 1986. Natural fractals in random structures like thin films, clouds and percolating clusters are used in understanding the material growth and morphology. An elementary first order electromagnet (EM) theory was used to elucidate the fractal screen by perforating an infinitely large, infinitesimally thin and perfectly conducting sheet by identical, small circular apertures.
Although the mathematics of fractals has been known for most of the twentieth century, the application of the fractal patterns to antenna technology is relatively new. The subject of fractal electrodynamics has been addressed in the references, On Fractal Electrodynamics, Recent Advances in Electromagnetic Theory, pages 183-224, 1990; Fractal Electrodynamics: Wave Interactions With Discretely Self Similar Structures, Electromagnetic Symmetry, pages 231-280, 1995; An Overview of Fractal Electrodynamics Research, Proceedings of the 11th Annual review of Progress in Applied Computational Electromagnetics, pages 964-969, 1995; Fractal Constructions of Linear and Planar Arrays, Proceedings of 1997 IEEE Symposium, pages 1968-1971, 1997; and On the Synthesis of Fractal Radiation Patterns, Radio Science, Vol. 30, pages 29-45, 1995.
Antennas with fractal patterns disposed on relatively low dielectric (dielectric constant of 2 to 3) substrates have been reported in the references, Fractal Antenna Applications in Wireless Telecommunications, Professional Program Proceedings of the electronics Industries Forum, pages 43-49, 1999 and Fractal Multiband Antenna Based on Sierpinski Gasket, IEEE Transactions Antennas Propagation, Vol. AP-46, pages 517-524, 1998. These references show that various fractal antennas improve the features of a conventional monopole antenna. However, to the best of the knowledge of the inventors, there is no study available to the effect of dielectric constant of the substrates in the performance of fractal antennas.
U.S. Pat. No. 4,948,922 describes an absorbent material comprised of a chiral substance.
U.S. Pat. No. 5,557,286 describes an antenna with a barium strontium titanate (BST) ceramic and a capability to tune the dielectric constant of the BST material. A copending United States patent application, Ser. No. 09/595,933, describes a tunable dual-band antenna having a BST material. However, neither the aforementioned patent nor application describes an antenna with a fractal pattern.
Antennas with the capability to change their radiation characteristics or operational frequency adaptively are generally classified as reconfigurable antennas. Reconfigurable antennas have been conventionally pursued for satellite communication applications, where it often is required to change the broadcast coverage patterns to suit the traffic changes. Reconfigurable antennas also find applications in a modern telecommunications scenario, where the same antenna could be shared between various functions (requiring frequency switching), or the antenna radiation characteristics could be altered as done in smart antennas, using signal processing techniques. In addition, reconfigurable antenna systems can also find applications in collision avoidance radars.
An antenna of the present invention has a substrate with a dielectric constant of at least 10 with an electrically conductive layer comprising a fractal pattern. A body or sheet of electrically conductive material is provided as a ground plane. A bias voltage is applied across the substrate to tune the antenna for operation in at least one frequency band. Input energy is fed via an input feed to the fractal pattern layer. The fractal pattern may be any suitable fractal pattern, such as Hilbert curve, Koch curve, Sierpinski gasket and Sierpinski carpet.
The antenna of the invention is capable of operation across an extremely large portion of the frequency spectrum including frequencies in the MHz range to frequencies in the GHz range. Also, the antenna can be constructed in a miniature size measured in centimeters compared to prior art antennas of the same class that have a size measured in meters. Also, the antenna is capable of being constructed in shapes that conform to a surface of an object, such as clothing, a vehicle, and the like.
In one class of embodiments of the invention, the ground plane is disposed substantially perpendicular to the substrate. In another class of embodiments of the invention, the ground plane is disposed substantially parallel to the substrate.
In some embodiments of the invention, the substrate is comprised of a ferroelectric material, which is preferably barium strontium titanate.
In some embodiments of the invention, a layer of absorbing material overlies a surface of the substrate opposite to the fractal pattern. The absorbing material layer smoothens the frequency/return loss characteristic of the antenna, thereby improving the wide band operation thereof. Preferably, the absorbing material is a chiral material.
In some embodiments of the antenna of the present invention, the dielectric constant is in the range of about 10 to about 200. In other embodiments the dielectric constant is in the range of about 200 to 600.
An alternative embodiment of the antenna of the present invention comprises first and second assemblies that each has a substrate of dielectric material having a first surface and a second surface and a fractal pattern electrically conductive layer that overlies the first surface of the substrate. A layer of absorbing material is disposed between the second surfaces of the first and second assemblies. A body or sheet of electrically conductive material is disposed in relation to the first and second assemblies so as to serve as a ground plane. In one style of this alternative embodiment, the ground plane is substantially perpendicular to the substrates and gives the antenna the capability of radiating energy in at least a hemispherical volume. In another style, the ground plane is disposed between and substantially parallel to the substrate so as to give the antenna the capability of radiating in substantially a spherical volume. This style of antenna has two absorbing layers, one disposed between the ground plane and one of the substrates and the other disposed between the ground plane and the other substrate.
In another alternative embodiment of the antenna of the present invention, an electrically conductive fractal pattern layer overlies a surface of a dielectric substrate. The fractal pattern has a plurality of segments arranged in a first configuration. One or more switches are disposed to change the first configuration to a second configuration. Preferably, the fractal pattern is a Hilbert curve. In some styles of this alternative embodiment, the dielectric substrate has a dielectric constant of at least 10. In other styles the dielectric constant is in the range of about 10 to about 200 or in the range of about 200 to about 600. The dielectric substrate may comprise a ferroelectric, which is preferably barium strontium titanate. Also, a bias voltage may be applied across the substrate for tuning purposes.
In another alternative embodiment of the invention, a plurality of fractal antennas are arranged in an array with a feed network that is capable of delivering signals thereto in a phased relation.